As an information society has been recently developed, an indoor stationary type of imaging apparatus has been required to have a larger display while for a mobile type apparatus, it has become essential that it can be used in both dark and bright places. Furthermore, weight reduction has been required for both types. As a result, a conventional CRT (Cathode Ray Tube) display has been replaced by a flat display.
Application of information devices have been expanded from indoor stationary types to mobile types. In contrast to stationary types, mobile type information devices are used in various situations. Stationary type devices are required to have a larger display and exhibit higher brightness and a wide viewing angle. Mobile type devices are required to exhibit stable visibility in both dark and bright places and improved impact resistance such as drop-impact resistance because they are used in a wide variety of situations.
Known flat displays include a plasma display, a liquid crystal display and an organic EL display (Organic Light Emitted Display). A plasma display is not suitable for a mobile device because it requires a high voltage from its operation principle, while a liquid crystal display and an organic EL display which can be operated with low power consumption are suitable for a mobile device.
A plasma display has been ahead as a large display because of its higher brightness and wide viewing angle. However, a liquid crystal display can be reduction in weight and, as a plasma display, large-sized. Thus, the liquid crystal display has been recently large-sized as a plasma display.
Meanwhile, in terms of a mobile device, a plasma display is not suitable for a mobile device because it requires a high voltage from its principle of operation, while a liquid crystal display and an organic EL display which can be operated with low power consumption are suitable for a mobile device.
Although liquid crystal displays prevail at present, it is expected that organic EL displays will be increased because of their clear picture.
Organic EL displays and liquid crystal displays are classified into “active driving types” where each pixel is equipped with an active device for driving, and “simple matrix types” where a pixel is driven by two groups of orthogonal electrodes. An active driving type can drastically reduce a response time in comparison with a simple matrix type, allowing a number of pixels to be used for movie displaying. Furthermore, it can more precisely control image-quality factors such as contrast and gradation, allowing a movie to be displayed in a quality comparable to a CRT. As a result, an “active driving type” is now a dominant driving system.
While a CRT or an organic EL display is a self-luminous type, a liquid crystal display develops colors using a transmitted or reflected light. Liquid crystal displays can be classified into three groups, i.e., transmission, reflection and semi-transmissive types, in which a pixel electrode transmits, reflects or partially transmits and partially reflects a light, respectively.
When a device is exclusively for indoor use as a stationary type, an image is clear in a transmissive liquid crystal display or an organic EL display. However, image contrast is deteriorated in an outdoor area which is brighter than emission intensity of natural light, leading to an obscure image. When a light source is intensified for preventing contrast deterioration outside, dazzling occurs in an indoor image and power consumption is increased.
In contrast, a reflective liquid crystal display has an advantage of higher outdoor visibility because it reflects an outside light to display an image, but has a drawback that an image is obscure in a dark place. Although the problem can be improved by incorporating a front light, a front light has a drawback that it cannot evenly illuminate the whole display even for a small display as in a mobile device.
There is a semi-transmissive liquid crystal display as a liquid crystal display with advantages of the transmission and the reflective types. A semi-transmissive liquid crystal display utilizes both backlight and outside light for displaying, by making a pixel electrode semi-transparence or forming an opening, ensuring visibility in both outdoor and indoor places. In most of mobile information terminals, semi-transmissive liquid-crystal panels are used at present.
However, an image in a semi-transmissive liquid crystal display is inferior to that in a transmissive liquid crystal display or organic EL display in a dark place and inferior to that in a reflective liquid crystal display in a bright place. Therefore, it is necessary to further improve image quality for using it as a mobile information terminal.
Furthermore, displays are used in a wide variety of private and commercial applications including information terminals such as mobile devices, e.g., a cell phone and a PDA (Personal Digital Assistant), digital cameras and digital video cameras, which are used in various places. Thus, such display apparatus are required to be robust.
Properties needed in a display panel for a mobile device include, in addition to image quality described above, a display size, panel thinness and power consumption.
Robustness leads to a thinner panel. Furthermore, it is necessary to use a substrate resistant to impact. In terms of a thickness of a panel, an organic EL display can be thinned to a thickness of one substrate in principle. In contrast, for a liquid crystal display panel, a reflective liquid crystal display can be thinned to a thickness of two substrates, while a transmission/semi-transmission type liquid crystal display inevitably becomes thicker because it requires a backlight.
There will be discussed factors other than image quality, including an overall dimension, a weight, robustness, power consumption and a price. In terms of an overall dimension and a weight, an organic EL display is overwhelmingly advantageous, which does not need a light source such as a backlight. It can be theoretically thinned and weight-reduced substantially to a level of one supporting substrate by effective sealing technique. Without incorporating an auxiliary light source, a reflective liquid crystal display panel can be thinned and weight-reduced to a level of two supporting substrates. However, it is still less advantageous in comparison with an organic EL display.
These devices exhibit substantially equivalent robustness as long as the same substrate is used. A reflective liquid crystal display is advantageous in terms of power consumption. However, when it is equipped with an auxiliary light source, it is comparable to a transmissive liquid crystal display or organic EL display.
In a semi-transmissive liquid crystal display, power consumption can be reduced in a bright place by turning a backlight off. Furthermore, a liquid crystal display has a longer history of commercial production than an organic. EL display, and therefore more advantageous in terms of a price.
In a liquid crystal display with the best performance, visibly perceptible fineness and the number of colors have substantially reached the upper limit. Further improvement in image quality is, therefore, insignificant. Thus, current research activities have been also focused on improvement performance factors other than image quality.
For example, by low-temperature polysilicon thin-film transistor (poly-Si TFT) technique, an electronic circuit which has been conventionally provided as an external device can be integrated on a glass substrate. Thus, in a liquid crystal display, the number of parts has been reduced, a frame has been narrowed and power consumption has been reduced. Studies on a liquid crystal display comprising a plastic substrate in place of a conventional glass substrate have been conducted, pursuing film thinning, weight-reduction and toughness to falling.
An organic EL display may be more promising than a liquid crystal display in terms of thinness, weight reduction and higher visibility, and thus have been studied for improvement in emission efficiency and a life.
As described above, a plasma display, a transmissive liquid crystal display or an organic EL display are suitable for stationary applications while a semi-transmissive liquid crystal display is suitable for mobile applications.
In the light of suitability for both stationary and mobile applications, it can be found that a liquid crystal display has advantages which a plasma display or organic EL display does not have.
FIG. 27 shows a cross-sectional view of a conventional semi-transmissive liquid crystal display panel. A liquid-crystal panel has a configuration that a liquid crystal is sandwiched by two substrates as shown in the upper par of FIG. 27. On one side of one glass substrate 312, there are regularly arranged pixels comprising a thin-film transistor 311 and a pixel electrode 310, and there is formed an interconnection for transferring a signal for driving the thin-film transistor 311. The pixel electrode 310 is designed to have a transmittance of 30 to 70%; often a transmittance of 70%.
On one side of the other glass substrate 304, there is arranged a color filter 305. The color filter (CF) 305 and a black matrix (BM) are disposed, facing a pixel electrode and a border between pixel electrodes, respectively, and covered by a transparent electrode 307. On the surfaces of these two substrates, there are formed oriented films 307, 309, respectively, for orienting a liquid crystal to a desired direction. These two substrates are fixed by a sealing material B disposed on the periphery of the substrates. A liquid crystal fills the space between these two substrates.
On the outer surfaces of each of these two substrates sandwiching the liquid crystal, there is attached a film substrate having various optical functions. In this figure, two film substrates, i.e., a polarizing plate (linear polarizing plate) 302, 314 and a retardation film (¼ wavelength plate) 303, 313, are laminated for converting an incident light into a circularly-polarized light. Furthermore, there is provided an antireflective plate 301 for preventing reflection of an outside light.
When applying the sealing material B, an opening is left for later injecting a liquid crystal. Spacers corresponding to a given space distance (for example, about 6 μm) are distributed for maintaining the given distance between these two substrates. The spacers are considerably smaller than a pixel electrode. After firing them under a certain load, a liquid crystal is injected from the opening in the sealing material. Finally, the opening in the sealing material is sealed with a UV-curable material to provide a liquid-crystal panel.
The lower part of FIG. 27 shows a configuration of a backlight.
The backlight comprises a light source C emitting a white light such as a lamp and a light-emitting diode (LED), an optical guide 317, a reflection plate 318, a diffusion sheet 316 and a field-angle regulating sheet 315.
A configuration of these components is optimized to allow the backlight to operate as a plane light emitter as even as possible and to guide a light from the light source C toward a liquid-crystal panel as efficiently as possible. In general, an optical guide is a transparent plastic substrate made of polymethyl methacrylate (PMMA) with a thickness of, for example, about 1.0 mm. The reflection plate 318, the diffusion sheet 316 and the field-angle regulating sheet 315 have been processed to effect individual optical functions. Thus, the overall thickness of the backlight components in FIG. 27 is about 2.0 mm.
There will be described operation of a semi-transmissive liquid crystal display as a transmissive liquid crystal display with reference to FIG. 27.
A white light from the light source C enters the optical guide 317, alters its path by the reflection plate 318 and then is diffused by the diffusion sheet 316. The diffused light is adjusted by the field-angle regulating sheet 315 to have a desired orientation and then reaches the liquid-crystal panel.
Although this light is non-polarized, only one linearly-polarized light passes through the straight polarizing plate 314 in the liquid-crystal panel. The linearly-polarized light is converted into a circularly-polarized light by the retardation film (¼ wavelength plate) 313, and sequentially passes through the glass substrate 312, the pixel electrode 310 made of a semi-transparent material, finally to the liquid crystal layer 308.
Orientation of the liquid crystal molecules are controlled, depending on the presence of a potential difference between the pixel electrode 310 and the opposite transparent electrode (counter electrode) 306. That is, in an extreme orientation state, a circularly-polarized light entering from the lower part of FIG. 27 is transmitted, as it is, through the liquid crystal layer 308 and then through the transparent electrode 306. Then, a light with a particular wavelength is transmitted through the color filter 305 to the retardation film (¼ wavelength plate) 313. Thus, it substantially completely passes through the polarizing plate (straight polarizing plate) 314. The pixel, therefore, most brightly displays a color determined by the color filter.
In contrast, in another extreme orientation state, polarity of a light passing through the liquid crystal layer is altered, so that a light passing through the color filter is substantially completely absorbed by the retardation film (¼ wavelength plate) 303 and the polarizing plate (straight polarizing plate) 302. Thus, the pixel displays black color. In an intermediate orientation state between these two states, a light is partially transmitted, so that the pixel displays an intermediate color.
Next, there will be described operation of a semi-transmissive liquid crystal display as a reflective liquid crystal display.
When an outside light enters a liquid-crystal panel from the upper part of FIG. 27, a circularly-polarized light which has been transmitted through the polarizing plate (straight polarizing plate) 302 and the retardation film (¼ wavelength plate) 303, passes through a liquid crystal layer. Then, 30% of the light is reflected by a pixel electrode to be utilized for displaying. Therefore, the display operates as a reflective liquid crystal display.
Next, there will be described operation of a semi-transmissive liquid crystal display.
In a semi-transmissive liquid crystal display, a pixel electrode is made of a semi-transparent material and its operation as a transmissive liquid crystal display is as described above, although when designing a light transmittance in the pixel electrode to, for example, 70%, 30% of the light is not used for displaying. On the other hand, when an outside light enters the liquid-crystal panel from the upper part of FIG. 27, a circularly-polarized light which has been transmitted through the straight polarizing plate and the ¼ wavelength plate passes through a liquid crystal layer and 30% of the light is reflected to be utilized for displaying. It, therefore, operates as a reflective liquid crystal display.
In the prior art, a substrate constituting a thin-film transistor has been a glass substrate which can tolerate an high-temperature during manufacturing the thin-film transistor. On the other hand, there has been investigated technique for forming a thin-film transistor at a lower temperature. For such technique, device properties are not adequate for forming a functional device on the substrate on which a thin-film transistor for driving a liquid crystal is formed, in response to the recent need for size reduction. Thus, the technique has not been practically used.
Thin-film transistors can be classified into three categories: high-temperature polysilicon transistors formed on a quartz substrate, low-temperature polysilicon transistors formed on a glass substrate and amorphous silicon transistors formed on a glass or plastic substrate. For size-reduction of a liquid-crystal panel, it has been attempted to form a driver IC which has been conventionally an external device, on a glass substrate. An amorphous silicon transistor can be manufactured at a lower temperature, but its properties adequate to operate a driver IC cannot be practically achieved on a plastic substrate. It is, therefore, more practical to form a low-temperature polysilicon transistor on a glass substrate, in the current manufacturing technique.
As shown in FIG. 28, a transmissive liquid crystal display utilizing a plastic substrate has been experimentally manufactured as described by Asano et al. (A. Asano and T. Kinoshita, “Low-temperature polysilicon TFT color LCD panel made of plastic substrates,” in Society for Information Display International Symposium Digest of Technical Papers (Society for Information Display, Boston, 2002,) Vol. 33, pp. 1196-1199.).
According to the above paper, on a glass substrate comprising an antietching layer is formed, by a well-known process for manufacturing a low-temperature polysilicon thin-film transistor, a polysilicon TFT, on which is then applied a removable adhesive, through which is then glued a temporary substrate (FIG. 28(a)). Next, the glass substrate is etched off by hydrofluoric acid (HF) (FIG. 28(b)). Then, after removing the antietching layer, a polysilicon TFT is glued via an adhesive to a plastic substrate with a thickness of 0.2 mm (FIG. 28(c)). Subsequently, the temporary substrate and the removable adhesive are sequentially removed (FIG. 28(d)). Next, the substrate and a substrate comprising, for example, a color filter and a transparent electrode are disposed, facing each other. Liquid crystal molecules are injected into the space between the substrates to form an active driving liquid crystal display panel.
A conventional transmissive/semi-transmissive liquid crystal display is thick and heavy because it uses a backlight. For solving the problem, there has been proposed a configuration using an organic EL.
JP-2000-29034-A and JP-2002-98957-A have disclosed a configuration using an organic EL as a backlight, which will be described below with reference to FIG. 29.
JP-2000-29034-A shown in FIG. 29A has described that in order to prevent an organic EL from being deteriorated due to a high temperature during forming an oriented film by a conventional firing process, an oriented film 623 which has been preliminarily oriented is laminated with a display-driving substrate 621 and a counter substrate 622.
The liquid crystal display panel in FIG. 29A is manufactured as follows. First, a TFT array substrate 621 and a counter substrate 622 comprising a plane light emitter which is produced by separate processes are laminated. Then, the product is subject to common rubbing to provide the polymer film with an orientating function to the liquid crystal composition 624 to form an oriented film 623. Then, the TFT array substrate 621 and the oriented film 623 to the counter substrate 622 are disposed, facing each other. Then, the space between them is filled with a liquid crystal composition 624.
In the structure in FIG. 29A, an organic film is laminated with the oriented film according to the prior art as shown in FIG. 27, and a backlight is replaced with an organic EL. Although a substrate for forming the organic EL is needed, a glass substrate has a thickness of 0.4 mm while a conventional optical guide has a thickness of several mm. Thus, it results in reduction in a film thickness.
JP-2000-98957-A has disclosed that in a transmissive liquid-crystal panel, an organic EL light-emitting device is used in place of a conventional fluorescent tube as a backlight for reducing a film thickness and a weight. FIG. 29B shows its structure.
The liquid crystal display panel comprises a first electrode substrate 650, a second electrode substrate 660 and a liquid crystal layer between these substrates.
The first electrode substrate 650 is comprised of a transparent glass substrate 651, whose surface to be in contact with a liquid crystal layer, comprises a scan line 652, a signal line 653 (not shown), a pixel electrode 654, a TFT 655, an auxiliary capacity 656 (not shown) and an auxiliary capacity line 657.
In the second electrode substrate 680, a transparent glass substrate 681 has a surface to be in contact with a liquid crystal on which a transparent electrode 682 to be a counter electrode to a liquid crystal and a surface facing the surface comprising substrate transparent electrode 682 in the glass substrate 681 comprises emitting parts 683, 685, 687, 689 in an organic EL and non-emitting parts 684, 686, 688 as spaces between the emitting parts 683, 685, 687, 689.
FIG. 29B shows that a film thickness can be reduced by eliminating an optical guide for a backlight which has been required in the prior art, by means of forming a plane light-emitting device consisting of an organic EL on the rear surface of the substrate comprising a counter electrode in a liquid-crystal panel. Thus, the number of substrates can be reduced to two while the conventional configuration needs three substrates as shown in FIG. 29A, resulting in thickness reduction in a liquid-crystal panel.
JP-54-126559-A has disclosed the use of a long flexible film as a substrate in a liquid-crystal panel. The application has, however, disclosed only an example where a long flexible film comprising a transparent electrode and an oriented film is used to form a simple matrix driving type of a monochrome liquid-crystal panel. The technique disclosed in JP-54-126559-A is related to manufacturing a liquid-crystal panel using a long plastic substrate in the era when a large and flat glass substrate was expensive and could not be easily produced. Furthermore, JP-62-150218-A and JP-06-27448-A have disclosed that a liquid crystal fills a space between long flexible films in which oriented films are formed on two electrodes.
However, a liquid-crystal panel had been required to be colorized and to display a movie. In order to increase a response speed, active matrix driving has been dominant as a driving system, where pixels in a liquid crystal are directly driven by a thin-film transistor. Furthermore, color displaying needs, in addition to an oriented film, other optically functional films such as a retardation film and a polarizing film. Additionally, as improvement in display visibility has been needed, there have been developed various types of liquid-crystal panels such as transmissive, reflective and semi-transmissive panels. Thus, the above technique could not respond to these.
Although there were problems of size increase and flatness in a glass substrate around 1975, these problems have been overcome by improvement in manufacturing technique for a glass substrate, and it is believed that a glass substrate is optimal for an active driving type liquid-crystal panel using a thin-film transistor.
In various types of liquid-crystal panels such as transmissive, reflective and semi-transmissive types, it is necessary to laminate a plurality of optically functional films with a substrate. In this process, the films are laminated one by one with the liquid-crystal panel, so that it requires many steps of laminating the optically functional films.
For simplifying the lamination process, JP-2002-358024-A and JP-2002-148607-A have disclosed that a long flexible film is laminated with a glass substrate.
Patent document 1 JP-2000-29034-A
Patent document 2: JP-2002-98957-A
Patent document 3 JP-54-126559-A
Patent document 4 JP-62-150218-A
Patent document 5: JP-2002-358024-A
Patent document 6: JP-2002-148607-A
Nonpatent literature 1: A. Asano and T. Kinoshita, “Low-temperature polysilicon TFT color LCD panel made of plastic substrates,” in Society for Information Display International Symposium Digest of Technical Papers (Society for Information Display, Boston, 2002,) Vol. 33, pp. 1196-1199.